There are automobiles in which an antenna for radiating radar waves and receiving reflected waves is mounted at the front nose or in the vicinity of the rear gate. However, these parts of the automobiles are the first to be deformed or damaged in cases of collisions with other vehicles or objects, even if the collisions are minor ones, and a radar mounted on such parts is also highly likely to be damaged. The radar is a device that is necessary to ensure the safety of automobiles, and it is not desirable for the radar to lose its functionality due to minor collisions. This is all the more so if automatic driving is put into practical use.
Such undesirable situations are less likely to occur if the radar device is mounted in the interior of a vehicle, but in that case, the radar device needs to transmit and receive radar waves through the windshield including glass. In this case, the reflection and absorption of the waves by the glass are unavoidable, and the radar will have limited detection capabilities.
European Patent No. 888646 discloses a method in which, when a communication antenna is installed in the interior of a vehicle, an intermediate dielectric member is disposed between glass and the radiating surface of the antenna in order to suppress the reflection of a radio wave by the glass. According to European Patent No. 888646, the electrically effective distance between the glass and the antenna is adjusted to several times the half-wavelength of the wave.
Incidentally, the thickness of the glass affects reflection from the entire glass, the reflection being an overlap of reflected waves from the front surface of the glass and from the rear surface of the glass. However, it is not usually possible to freely select the thickness of the glass of the windshield. Thus, the influence of the reflected wave from the rear surface of the glass has not been considered thus far.